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  SQL从删库到跑路(一)
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      <h1 id="一、什么是SQL？"><a href="#一、什么是SQL？" class="headerlink" title="一、什么是SQL？"></a>一、什么是SQL？</h1><blockquote>
<p><strong>SQL</strong>(Structured Query Language)<strong>结构化查询语言</strong></p>
</blockquote>
<blockquote>
<p>即定义了操作所有关系型数据库的规则。</p>
</blockquote>
<h1 id="二、SQL通用语法"><a href="#二、SQL通用语法" class="headerlink" title="二、SQL通用语法"></a>二、SQL通用语法</h1><ol>
<li>SQL语句可以单行或多行书写，以分号结尾</li>
<li>不区分大小写</li>
<li>注释</li>
</ol>
<ul>
<li><blockquote>
<p>单行注释：– 内容 或 # 内容</p>
</blockquote>
</li>
<li><blockquote>
<p>多行注释： /* 注释 */</p>
</blockquote>
</li>
</ul>
<h1 id="三、SQL分类"><a href="#三、SQL分类" class="headerlink" title="三、SQL分类"></a>三、SQL分类</h1><h2 id="1-DDL"><a href="#1-DDL" class="headerlink" title="1. DDL"></a>1. <strong>DDL</strong></h2><p> (Data Definition Language) <strong>数据定义语言</strong></p>
<ul>
<li><blockquote>
<p> 用来定义数据库对象：数据库，表，列等。关键字：Create,drop,alter等</p>
</blockquote>
</li>
</ul>
<h2 id="2-DML"><a href="#2-DML" class="headerlink" title="2. DML"></a>2. <strong>DML</strong></h2><p> (Data Manipulation Language)<strong>数据操作语言</strong></p>
<ul>
<li><blockquote>
<p>用来对数据库中的表数据进行增删改。关键字：insert，delete，update等</p>
</blockquote>
</li>
</ul>
<a id="more"></a>
<h2 id="3-DQL"><a href="#3-DQL" class="headerlink" title="3. DQL"></a>3. <strong>DQL</strong></h2><p>(Data Query Language)<strong>数据查询语言</strong></p>
<ul>
<li><blockquote>
<p>用来查询数据库中表的数据。关键字：select,where等</p>
</blockquote>
</li>
</ul>
<h2 id="4-DCL"><a href="#4-DCL" class="headerlink" title="4. DCL"></a>4. <strong>DCL</strong></h2><p>(Data Control Language)<strong>数据控制语言</strong></p>
<ul>
<li><blockquote>
<p>用来定义数据库的访问权限和安全级别，及创建用户。关键字：GRANT,REVOKE等 </p>
</blockquote>
</li>
</ul>
<h1 id="四、-DDL"><a href="#四、-DDL" class="headerlink" title="四、 DDL"></a>四、 DDL</h1><h2 id="1-操作数据库"><a href="#1-操作数据库" class="headerlink" title="1. 操作数据库"></a>1. 操作数据库</h2><ul>
<li><p>创建数据库</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">create database 数据库名;</span><br></pre></td></tr></table></figure>
</li>
<li><p>查询所有数据库的名称</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">show databases;</span><br></pre></td></tr></table></figure>
</li>
<li><p>查询某个数据库的字符集：查询某个数据库的创建语句</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">show create database 数据库名称;</span><br></pre></td></tr></table></figure>

</li>
</ul>
<p><img src="https://yolin121.oss-cn-beijing.aliyuncs.com/blogimg/Snipaste_2020-09-06_19-30-33.jpg" alt=""></p>
<ul>
<li><p>在不存在的情况下创建数据库</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">create database <span class="keyword">if</span> not exists 数据库名;</span><br></pre></td></tr></table></figure>
</li>
<li><p>按特定字符集创建数据库</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">create database 数据库名 charactor set gbk;</span><br></pre></td></tr></table></figure>

</li>
</ul>
<p><img src="https://yolin121.oss-cn-beijing.aliyuncs.com/blogimg/Snipaste_2020-09-06_19-37-51.jpg" alt=""></p>
<ul>
<li><p>将数据库字符集从gbk改为utf8格式</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">alter database 数据库名 character set utf8;</span><br></pre></td></tr></table></figure>
</li>
<li><p>删除数据库</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">drop database 数据库名;</span><br></pre></td></tr></table></figure>
</li>
<li><p>判断数据库是否存在，存在则删除，避免语法错误出现异常</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">drop database <span class="keyword">if</span> exists 数据库名称;</span><br></pre></td></tr></table></figure>
</li>
<li><p>查询当前使用的数据库名称</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="function">select <span class="title">database</span><span class="params">()</span></span>;</span><br></pre></td></tr></table></figure>
</li>
<li><p>使用数据库</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">use 数据库名;</span><br></pre></td></tr></table></figure>

</li>
</ul>
<p><img src="https://yolin121.oss-cn-beijing.aliyuncs.com/blogimg/Snipaste_2020-09-06_20-10-47.jpg" alt=""></p>
<h2 id="2-操作表"><a href="#2-操作表" class="headerlink" title="2. 操作表"></a>2. 操作表</h2><ul>
<li><p>查询某个数据库中所有的表名称</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">show tables;</span><br></pre></td></tr></table></figure>
</li>
<li><p>查询表结构</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">desc 表名</span><br></pre></td></tr></table></figure>

</li>
</ul>
<p><img src="https://yolin121.oss-cn-beijing.aliyuncs.com/blogimg/Snipaste_2020-09-06_20-16-48.jpg" alt=""></p>
<ul>
<li>创建表<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">create table 表名(</span><br><span class="line">   列名<span class="number">1</span> 数据类型，</span><br><span class="line">   列名<span class="number">2</span> 数据类型，</span><br><span class="line">   。。。</span><br><span class="line">   列名n 数据类型n</span><br><span class="line">);</span><br></pre></td></tr></table></figure>

</li>
</ul>
<h4 id="数据类型"><a href="#数据类型" class="headerlink" title="数据类型"></a>数据类型</h4><ol>
<li><strong>int</strong> : 整数类型</li>
<li><strong>double</strong> : 小数类型</li>
<li><strong>date</strong> : 日期，只包含年月日，yyyy-MM-dd</li>
<li><strong>datetime</strong> : 日期，包含年月日时分秒</li>
<li><strong>timestamp</strong> : 时间戳类型 包含年月日时分秒</li>
</ol>
<ul>
<li><blockquote>
<p><strong>如果将来不给这个字段赋值，或赋值为null，则默认使用当前的系统时间，来自动赋值</strong></p>
</blockquote>
</li>
</ul>
<ol start="6">
<li>varchar：字符串</li>
</ol>
<p><img src="https://yolin121.oss-cn-beijing.aliyuncs.com/blogimg/Snipaste_2020-09-06_20-58-49.jpg" alt=""></p>
<h4 id="在mydb中创建student表"><a href="#在mydb中创建student表" class="headerlink" title="在mydb中创建student表"></a>在mydb中创建student表</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="function">create table <span class="title">student</span><span class="params">(</span></span></span><br><span class="line"><span class="function"><span class="params">  id <span class="keyword">int</span>,</span></span></span><br><span class="line"><span class="function"><span class="params">  name varchar(<span class="number">20</span>)</span>,</span></span><br><span class="line"><span class="function">  score <span class="title">double</span><span class="params">(<span class="number">4</span>,<span class="number">1</span>)</span>,</span></span><br><span class="line"><span class="function">  birthday date,</span></span><br><span class="line"><span class="function">  insert_time timestamp</span></span><br><span class="line"><span class="function">  )</span>;</span><br></pre></td></tr></table></figure>

<p><img src="https://yolin121.oss-cn-beijing.aliyuncs.com/blogimg/Snipaste_2020-09-06_21-10-22.jpg" alt=""></p>
<ul>
<li>删除表<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">drop table 表名;</span><br><span class="line">drop table <span class="keyword">if</span> exists 表名;</span><br></pre></td></tr></table></figure>

</li>
</ul>
<p><img src="https://yolin121.oss-cn-beijing.aliyuncs.com/blogimg/Snipaste_2020-09-06_21-20-28.jpg" alt=""></p>
<ul>
<li><p>修改表名</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">alter table 表名 rename to 新的表名;</span><br></pre></td></tr></table></figure>
</li>
<li><p>修改表的字符集</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">alter table 表名 charactor set 字符集;</span><br><span class="line"><span class="comment">//查看表的字符集</span></span><br><span class="line">show create table 表名;</span><br></pre></td></tr></table></figure>
</li>
<li><p>添加一列</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">alter table 表名 add 列名 数据类型</span><br></pre></td></tr></table></figure>
</li>
<li><p>修改列名称 类型</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">alter table 表名 change 列名 新列名 新数据类型;</span><br><span class="line"></span><br><span class="line">alter table 表名 modify 列名 新数据类型;</span><br></pre></td></tr></table></figure>
</li>
<li><p>删除列</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">alter table 表名 drop 列名;</span><br></pre></td></tr></table></figure>

</li>
</ul>
<p><img src="https://yolin121.oss-cn-beijing.aliyuncs.com/blogimg/Snipaste_2020-09-06_21-44-45.jpg" alt=""></p>

      
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</script>


<script src="/js/ayer.js"></script>


<script src="https://cdn.jsdelivr.net/npm/jquery-modal@0.9.2/jquery.modal.min.js"></script>
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/jquery-modal@0.9.2/jquery.modal.min.css">


<!-- Root element of PhotoSwipe. Must have class pswp. -->
<div class="pswp" tabindex="-1" role="dialog" aria-hidden="true">

    <!-- Background of PhotoSwipe. 
         It's a separate element as animating opacity is faster than rgba(). -->
    <div class="pswp__bg"></div>

    <!-- Slides wrapper with overflow:hidden. -->
    <div class="pswp__scroll-wrap">

        <!-- Container that holds slides. 
            PhotoSwipe keeps only 3 of them in the DOM to save memory.
            Don't modify these 3 pswp__item elements, data is added later on. -->
        <div class="pswp__container">
            <div class="pswp__item"></div>
            <div class="pswp__item"></div>
            <div class="pswp__item"></div>
        </div>

        <!-- Default (PhotoSwipeUI_Default) interface on top of sliding area. Can be changed. -->
        <div class="pswp__ui pswp__ui--hidden">

            <div class="pswp__top-bar">

                <!--  Controls are self-explanatory. Order can be changed. -->

                <div class="pswp__counter"></div>

                <button class="pswp__button pswp__button--close" title="Close (Esc)"></button>

                <button class="pswp__button pswp__button--share" style="display:none" title="Share"></button>

                <button class="pswp__button pswp__button--fs" title="Toggle fullscreen"></button>

                <button class="pswp__button pswp__button--zoom" title="Zoom in/out"></button>

                <!-- Preloader demo http://codepen.io/dimsemenov/pen/yyBWoR -->
                <!-- element will get class pswp__preloader--active when preloader is running -->
                <div class="pswp__preloader">
                    <div class="pswp__preloader__icn">
                        <div class="pswp__preloader__cut">
                            <div class="pswp__preloader__donut"></div>
                        </div>
                    </div>
                </div>
            </div>

            <div class="pswp__share-modal pswp__share-modal--hidden pswp__single-tap">
                <div class="pswp__share-tooltip"></div>
            </div>

            <button class="pswp__button pswp__button--arrow--left" title="Previous (arrow left)">
            </button>

            <button class="pswp__button pswp__button--arrow--right" title="Next (arrow right)">
            </button>

            <div class="pswp__caption">
                <div class="pswp__caption__center"></div>
            </div>

        </div>

    </div>

</div>

<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe.min.css">
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/default-skin/default-skin.css">
<script src="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe-ui-default.min.js"></script>

<script>
    function viewer_init() {
        let pswpElement = document.querySelectorAll('.pswp')[0];
        let $imgArr = document.querySelectorAll(('.article-entry img:not(.reward-img)'))

        $imgArr.forEach(($em, i) => {
            $em.onclick = () => {
                // slider展开状态
                // todo: 这样不好，后面改成状态
                if (document.querySelector('.left-col.show')) return
                let items = []
                $imgArr.forEach(($em2, i2) => {
                    let img = $em2.getAttribute('data-idx', i2)
                    let src = $em2.getAttribute('data-target') || $em2.getAttribute('src')
                    let title = $em2.getAttribute('alt')
                    // 获得原图尺寸
                    const image = new Image()
                    image.src = src
                    items.push({
                        src: src,
                        w: image.width || $em2.width,
                        h: image.height || $em2.height,
                        title: title
                    })
                })
                var gallery = new PhotoSwipe(pswpElement, PhotoSwipeUI_Default, items, {
                    index: parseInt(i)
                });
                gallery.init()
            }
        })
    }
    viewer_init()
</script>



<script type="text/x-mathjax-config">
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          inlineMath: [ ['$','$'], ["\\(","\\)"]  ],
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      }
  });

  MathJax.Hub.Queue(function() {
      var all = MathJax.Hub.getAllJax(), i;
      for(i=0; i < all.length; i += 1) {
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<script src="https://cdn.jsdelivr.net/npm/mathjax@2.7.6/unpacked/MathJax.js?config=TeX-AMS-MML_HTMLorMML"></script>


<script type="text/javascript" src="https://js.users.51.la/20544303.js"></script>
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